Collagen connective tissue is ubiquitous in the human body and demonstrates several unique characteristics not found in other tissues. It provides the cohesiveness and tenacity of the musculo-skeletal system, the structural integrity of the viscera, as well as the elasticity of the integument.
Most endothelial-lined structures of the body have collagen cores for specific functional purposes. Collagen cores are found in structures as diverse as the trabecular meshwork of the aqueous filtration system of the eye, and the valves of the heart. The walls of the great vessels share their collagen integrity with the ligamentous bony attachments and the tendinous or sinewy muscular attachments to the long bones. The cornea of the eye is a unique example of collagen connective tissue with the cornea stroma (accounting for about 90% of the total thickness of the cornea) demonstrating a high transparency of cross-oriented individual sheets or lamellae of collagen with a high (about 70%) water content and lesser (about 8%) amounts of protein and muco-polysaccharides.
Intermolecular cross-links provide collagen connective tissue with unique physical properties of high tensile strength and substantial elasticity. The extracellular matrix of this tissue consists of complex macromolecules, the biosynthesis of which involves several specific reactions that are often under stringent enzymatic control. The cross-linking is mediated, for example, by the copper-dependent enzyme lysyl-oxidase, and can be inhibited by chemicals such as B-aminoproprionitrile, as well as by various types of energy such as heat and photonic radiation. The net accumulation of collagen connective tissue is then dependent upon the precise balance between the synthesis and degradation of the connective tissue components.
A previously recognized property of hydro-thermal shrinkage of collagen fibers when elevated in temperature to the range 60.degree. to 70.degree.C. (an increase of about 30.degree. C. above normal body temperature) is but one of the unique characteristics of this tissue not exhibited by other body tissues. Temperature elevation ruptures the collagen ultrastructural stabilizing cross-links, and results in immediate contraction in the fibers to about one-third of their original lineal dimension, while increasing the caliber of the individual fibers without changing the structural integrity of the connective tissue.
The present invention is directed to a method and apparatus for effecting controlled lineal contraction or shrinkage of collagen fibers to provide a multitude of nondestructive and beneficial structural changes and corrections within the body. The invention has application to the alteration of collagen connective tissue throughout the body, and will be described with specific reference to correction of refractive disorders of the cornea of the eye.
These applications have received some discussion in existing literature, but presently known techniques do not provide an adequate basis for effective use of this knowledge of the properties of collagen as a safe and predictable treatment method.
The cornea is a layered structure which provides the majority of the eye's refractive or focusing power for incoming light rays which are transmitted through the crystalline lens of the eye to light-sensitive receptors of the retina. The corneal layers, from outer to inner surfaces, include the epithelium, Bowman's membrane, a relatively thick central stroma formed of cross-oriented collagen ribbons or sheets, Descemet's membrane, and the endothelium. The as-yet unmet challenge is to achieve a thermal profile within the stroma to attain controlled, predictable collagen shrinkage and resulting corneal shape change and adjustment of refractive effects without damaging the adjacent layers.
An earlier approach to corneal reshaping to correct vision defects involved direct application of a heated probe to the corneal epithelium to transmit heat to the stromal collagen fibers. This technique, sometimes called thermokeratoplasty or TKP, was substantially unsuccessful in that peak temperatures were necessarily achieved in the outer corneal layers rather than in the stroma where the beneficial effect of collagen heating was desired. The most serious and discouraging problem was irreparable temperature damage to the corneal epithelium and its basement membrane, with consistent findings of thermal dissolution and persistent defects in this membrane. This has resulted in faulty epithelial adhesion and recurrent corneal epithelial erosions.
In contrast to corneal-stroma problems encountered in previous investigations, the desired method herein disclosed achieves highest shrinkage temperatures in the midstroma, and lowest in the region of Decemet's membrane and the endothelial monolayer on the inner surface of the cornea. The thermal profile must be controlled within a narrow peak range of 5.degree. to 7.degree. C. in order to destabilize the covalent bonding (or to disrupt interchain hydrogen bonds) of this triple-helical collagenous domain to achieve desired shrinkage, and without significantly traumatizing the keratocytes or denaturing the collagen fibrils. The thermal trauma associated with earlier efforts in this field leads to an acute inflammatory tissue response which results in the removal of denatured collagen, and is characterized by the deposition and subsequent cross-linking of newly elaborated collagen at the site as catalyzed by the enzyme lysyl oxidase.
The rapid replacement of contracted collagen fibers by new mature collagen following trauma results in the unwanted reversal of the desired corneal reconfiguration. In the absence of trauma, the half life of Type I collagen has been shown to be consistent with the life of the experimental animal.
Prior investigations, however, have not considered the importance of the atraumatic attainment of the proper thermal profile for protracted or permanent recurving of the cornea in the absence of collagen fibrillar replacement associated with trauma and the resulting inflammatory response.
Damage to the endothelial monolayer is the most disturbing problem encountered when the peak temperature is too far posterior in the cornea. Factors influencing the quality of this most important corneal layer include the absolute number of viable endothelial cells, and the morphology of these cells. Endothelial cells, unlike epithelial cells, are not replaced following trauma. There are several studies suggesting that cell shape (polymegathism and pleomorphism) is more closely related to the functional reserve of this layer than to endothelial cell density, but in either case complications will result in persistent edema, bullous keratopathy and loss of transparency of the cornea.
The problem of confining peak temperature to the stroma while maintaining acceptably lower temperatures in the inner and outer adjacent corneal layers is recognized in the prior art. U.S. Pat. Nos. 4,326,529 and 4,381,007, for example, disclose use of radio-frequency heating while irrigating the outer corneal surface with a cooling saline solution. Published reports on the technique, however, note ciliary spasm and fluctuating corneal power (topographic hysteresis) up to two months postoperatively. All patients had stroma scarring after the procedure, and the flattening induced was short lived.
The emergence of the laser as a practical tool for ophthalmologists has led to investigation of the use of coherent energy as a means for achieving corneal shape change to correct vision defects. One such application, essentially unrelated to the present invention, is disclosed in U.S. Pat. No. 4,461,294 which proposes the laser as a tissue-destructive (ablative photodecomposition) tool for forming radial corneal scars in a technique called radial keratotomy.
Use of the laser as a corneal collagen-shrinking tool has also been disclosed in the literature, but not in the context of a practical system which avoids tissue necrosis in the corneal epithelium, while providing predictable reconfiguration of the tissue without loss of transparency. The known technology thus does not disclose a procedure which avoids tissue necrosis, and produces protracted or permanent corneal recurving proportional to energy distribution, and repeatable (as indicated by animal studies) for similar exposure patterns and energy level.
The literature suggests that by properly selecting the absorption coefficient and using heat removal at the corneal surface, a proper temperature profile can be achieved in the cornea (high in the stroma and low in both the epithelium and endothelium). These studies conclude that the absorption coefficient must be in the range of 190 cm.sup.-1 for this to occur; this restricts the range of wavelength interest to 2.6 or 3.9 microns; and that no lasers are commercially available at those wavelengths. This conclusion that the proper thermal profile is solely wavelength dependent is incomplete and has discouraged investigation in other wavelength domains. It is further believed that earlier investigations have incorrectly assumed that the absorption coefficient of corneal stroma is closely approximated by the absorption coefficient of water.
The present invention recognizes that in addition to absorption coefficient and anterior surface heat removal, the footprint of the energy with time significantly influences the temperature profile in the cornea. Specifically, by using pulsed or burst mode energy, a proper temperature profile has been obtained at much lower absorption coefficients (15-120 cm.sup.-1), allowing use of lasers operating in the range of 1.80-2.55 micron wavelengths within today's technology. This method avoids the trauma of improper thermal profiles, and obtains proportional changes from at least 2 to 13 diopters in refractive power related to exposure pattern and energy density. This method has been shown to be repeatable in that similar changes in corneal curvature were observed for similar patterns and exposure levels. Significant induced effect has persisted throughout follow-up investigation, lending evidence that the half-life of corneal collagen was undisturbed.